Move the code here and there

This patch rearranges raft structures an their responsibilities. It enhances the "low coupling - high cohesion" balance.

The whole patch is divided into small atomic commits, each introducing a single change:

1. introduce intermediate InnerNode struct
2. access RawNode through InnerNode
3. move notifications code into InnerNode
4. encapsulate InnerNode initialization
5. move most of Node functions into InnerNode
6. retain RaftSpaceAccess in InnerNode
7. shorten errors text
8. move ready state handling into InnerNode
9. move conn_pool to the InnerNode
10. postpone proc termination during expel
11. move main_loop into the InnerNode